1. Field of the Invention
The present invention relates to a fuel cell system.
2. Discussion of the Background
A fuel cell vehicle (fuel cell movable body) has been rapidly developed, which includes a fuel cell such as a polymer electrolyte fuel cell (PEFC) and is driven by operating an electric drive motor with power generated by the fuel cell.
In such a fuel cell vehicle, to decrease fuel consumption, that is, to decrease consumption of hydrogen (reactant gas), for example, a technique has been suggested (see Japanese Unexamined Patent Application Publication No. 2006-294304), in which supply of hydrogen and air to a fuel cell is stopped (this state is called idle stop) if an idle state is continued because of waiting for signal and hence a predetermined condition is established.
The technique continues power generation of the fuel cell, i.e., continues discharge from the fuel cell even after the idle stop, and allows hydrogen and air remaining in the fuel cell to be consumed, thereby preventing the fuel cell from being left at a high voltage.
When the idle stop is started, if an electric current value of the fuel cell for discharge is set to a constant value regardless of a power-generation state of the fuel cell before the idle stop, the electric current value may markedly vary. Consequently, a voltage value of the fuel cell may also markedly vary. For example, in a case where a fuel cell, in which power is generated at a high current value, is subjected to idle stop, when the electric current value varies and becomes a low current value, a voltage value of the fuel cell is rapidly increased.
When the voltage value of the fuel cell rapidly varies, for example, an internal circuit is formed in a unit cell of the fuel cell. Current may flow through the internal circuit, resulting in the fuel cell (electrolyte membrane, electrode, catalyst, etc.) being degraded.